The present invention relates to a microwave cooking oven and more specifically to an improved arrangement for preventing contamination of a sensor used to sensing gaseous emissions from food during the cooking cycle.
It is now well known in the art to employ sensing devices responsive to gases or humidity in the air exiting the cooking cavity for controlling the duration of the cooking cycle. Representative schemes for using such sensing devices are described in U.S. Pat. No. 4,336,433 and U.S. Pat. No. 4,311,895. The former describes a system utilizing a humidity sensor and the latter describes a system using a gas sensor. Use of either type of sensing device requires that the device be exposed to the air exiting the cooking cavity. This exiting air carries various organic fatty compounds emitted from the food which tend to accumulate on the sensor so as to hinder its proper operation. For example, during the roasting of meats, fats in the vapor phase are released together with steam. These fats tend to collect on the sensor element producing an impermeable barrier to the water vapor or other gaseous constituents.
One approach, described in U.S. Pat. No. 4,080,564, employed with humidity sensors for such applications is to provide a heating resistor in close proximity to the sensor element which is periodically energized during the cooking cycle to burn off contaminants which have accumulated on the sensor surface. This arrangement satisfactorily cleans the sensor. However, it is necessary to suspend heating of the food in the cavity during the cleaning intervals since the sensor is inoperative during these intervals resulting in prolonged cooking cycles. It has been determined that for satisfactory operation, a cleaning interval of up to one minute duration may be necessary. In addition, the rate of contaminant accumulation, particularly for food items high in fat and starch content, may necessitate cleaning intervals as often as every 9 minutes for satisfactory operation. Thus, each hour of actual cooking time could require an additional 6-7 minutes of cleaning time.
One preventive approach employed with a gas sensor is to enclose the sensor within a fine mesh screen. However, over time a barrier of contaminants will accumulate on the screen, necessitating periodic removal for cleaning. Another approach, described in Japanese Pat. No. 55-112937, provides for mounting the sensor for periodic movement into the exit air path for sensing. This slows contaminant accumulation but would still require eventual cleaning. Also, the mounting arrangement adds cost and adversely impacts reliability.
It is apparent from the foregoing that an arrangement which prevents the airborne contaminants from interferring with sensor operation without interruption of the cooking cycle resulting in prolonged cycle times, and which does not require periodic cleaning of the sensor device by the user, is desirable.
It is a primary object of the present invention to provide in a microwave oven employing a sensor for detecting gaseous food emissions an improved sensor arrangement which protects the sensor from contaminant accumulation without interrupting or altering the cooking cycle and which does not require periodic user intervention.
It is a further object of the invention to provide in such an oven means upstream of the sensing device for converting airborne nongaseous contaminants to gaseous form to prevent accumulation of such contaminants on the sensing device.
It is a further object of the invention to provide in such an oven pyrolytic conversion means for removing by pyrolysis non-gaseous airborne contaminants from the air stream flowing over the sensor.
It is a further object of the invention to provide in an oven of the aforementioned type cooling means interposed between the pyrolytic conversion means and the sensor to cool the air heated by the pyrolytic means to a predetermined temperature before the air reaches the sensor.